STRUCTURAL AND THERMAL-PROPERTIES OF THE TETRAGONAL COBALT MANGANESE SPINELS MNXCO3-XO4 (1.4-LESS-THAN-X-LESS-THAN-2.0)

Phase transitions experienced by MnxCo3-xO4(1.4 < x < 2.0) materials s ynthesized at low temperature have been followed by X-ray powder diffr action and thermal analysis. A tetragonal single phase (space group I4 1/amd) has been isolated for the first time for samples quenched from T > 700 degrees C, with a distortion parameter ranging from 1.0263(1) for x = 1.45 to 1.1353(1) for x 1.9. The existence at T > 725 degrees C of a nonisolable cubic spinel single phase (space group Fd3m) in the whole compositional range has been shown by high-temperature X-ray di ffraction techniques. It readily transforms on quenching into the new tetragonal single phase. When the cubic high-temperature single phase is allowed to cool slowly, it decomposes into a mixture of tetragonal (x approximate to 2) and cubic (x approximate to 1.3) spinel-type phas es. The composition x = 1.4 is found to be the upper compositional lim it of existence of the cubic Fd3m spinel, when it is synthesized by a low-temperature procedure.